Method and apparatus for facilitating proximity communication and power delivery

ABSTRACT

The described embodiments provide a system that facilitates inter-chip alignment for proximity communication and power delivery. The system includes a first integrated circuit chip and a second integrated circuit chip, both of which whose surfaces have corresponding etch pit wells configured to align with each other. A shaped structure is placed in an etch pit well of the first integrated circuit chip such that when the corresponding etch pit well of the second integrated circuit chip is substantially aligned with the etch pit well of the first integrated circuit chip, the shaped structure mates with both the etch pit well of the first integrated circuit chip and with the corresponding etch pit well of the second integrated circuit chip, thereby aligning the first integrated circuit chip with the second integrated circuit chip. In some embodiments the etch pit wells include conductive structures for routing power through a conductive shaped structure.

RELATED APPLICATION

This application is a divisional application of, and hereby claimspriority under 35 U.S.C. § 120 to, pending U.S. patent application Ser.No. 11/385,445, filed 20 Mar. 2006, entitled “Method and Apparatus forFacilitating Proximity Communication and Power Delivery” (Atty. DocketNo. SUN05-0538-CIP1). The 11/385,445 patent application is acontinuation-in-part of pending U.S. patent application Ser. No.11/243,300 filed on 3 Oct. 2005 by inventors Ashok V. Krishnamoorthy,John E. Cunningham, and Edward L. Follmer, entitled “Method andApparatus for Precisely Aligning Integrated Circuit Chips” (AttorneyDocket No. SUN05-0538). This application hereby claims priority under 35U.S.C. § 120 to the 11/243,300 patent application.

GOVERNMENT LICENSE RIGHTS

This invention was made with United States Government support underContract No. NBCH3039002 awarded by the Defense Advanced ResearchProjects Administration. The United States Government has certain rightsin the invention.

BACKGROUND

1. Field of the Invention

The present invention relates generally to techniques for assemblingsystems containing integrated circuit chips or wafers. Morespecifically, the present invention relates to a method and an apparatusfor precisely assembling systems containing semiconductor chips tofacilitate proximity communication and power delivery between thesemiconductor chips.

2. Related Art

Conductive electrical interconnections and transceivers are generallyused to provide reliable communications between integrated circuit (IC)chips in computer systems, due to their packaging and manufacturingadvantages. However, decreasing semiconductor line-widths and increasingon-chip clock speeds are putting pressure on the ability of traditionalresistive wires to achieve the off-chip bandwidths necessary to fullyutilize on-chip computational resources.

A new technique referred to as “proximity communication” overcomes thelimitations of conductive connections by using capacitive coupling toprovide communications between chips which are oriented face-to-face.This capacitive coupling can provide signal densities two orders ofmagnitude denser than traditional off-chip communication usingwire-bonding or traditional ball-bonding, while the circuits andcoupling structures remain fully-compatible with standard CMOSfoundries. To communicate off-chip through capacitive coupling, on-chipcircuits drive high-impedance, capacitive transmitter pads. Suchcommunication avoids impedance conversion and thereby reduces the powernormally dissipated by off-chip driver circuits. Moreover, simple drivercircuits and small chip-to-chip distances can significantly reduce thetotal chip-to-chip communication latency.

While proximity communication provides off-chip signaling bandwidth thatscales with chip feature size, it also introduces topologicalconstraints. The active sides of chips typically need to face each otherwith full or partial overlap, so that corresponding transmitter andreceiver pads on opposing chips align both laterally and vertically.Achieving and maintaining chip alignment for proximity communication ischallenging, especially when combined with other constraints such asconductive connections for other signals such as power and ground.

Hence, what is needed is a method and an apparatus that facilitatesproximity communication and power delivery without the limitations ofexisting approaches.

SUMMARY

One embodiment of the present invention provides a system thatfacilitates precise inter-chip alignment for proximity communication andpower delivery. The system includes a first integrated circuit chip,whose surface has etch pit wells. The system also includes a secondintegrated circuit chip, whose surface has corresponding etch pit wellsconfigured to align with the etch pit wells of the first integratedcircuit chip. A shaped structure is placed in an etch pit well of thefirst integrated circuit chip such that when the corresponding etch pitwell of the second integrated circuit chip is substantially aligned withthe etch pit well of the first integrated circuit chip, the shapedstructure mates with both the etch pit well of the first integratedcircuit chip and with the corresponding etch pit well of the secondintegrated circuit chip, thereby precisely aligning the first integratedcircuit chip with the second integrated circuit chip. Besides aiding inalignment, the shaped structure also creates a conductive connectionbetween the first integrated circuit chip and the second integratedcircuit chip.

In a variation on this embodiment, the etch pit well includes conductivematerial that conducts signals to and from other regions of theintegrated circuit chip. This conductive material allows a signal, suchas power or ground, to be conducted between the first integrated circuitchip and the second integrated circuit chip through the shapedstructure.

In a further variation, the shaped structure can be formed in a varietyof different shapes that include: a substantially spherical ball; atruncated sphere; a prism; a hexagon; and/or a pyramid.

In a further variation, the system includes multiple shaped structureswith different sizes. These multiple shaped structures can conductdifferent signals or a smaller set of signals with fault toleranceand/or higher current capacity.

In a variation on this embodiment, the shaped structure is comprised ofeither a conductive material or a non-conductive material with aconductive coating.

In a variation on this embodiment, the system uses compressive forces toalign the first integrated circuit chip, the shaped structure, and thesecond integrated circuit chip. Compliance in the shaped structureand/or the etch pit wells facilitates surface contact between the shapedstructure and the sidewalls or bottom of the etch pit wells.

In a variation on this embodiment, the shaped structure is bonded to thefirst integrated circuit to create a male-female connector interface.

In a variation on this embodiment, the shaped structure provides aconnection that can be disconnected and reconnected easily.

In a further variation, a conductive material that reduces contactresistance is placed inside the etch pit well to increase surfacecontact area and reduce electrical resistance. This conductive materialcan include a conductive low-viscosity paste or liquid.

In a variation on this embodiment, the etch pit well and the shapedstructure are formed using: an additive process; a subtractive process;or both an additive and subtractive process.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an array of chips communicating using proximitycommunication in accordance with an embodiment of the present invention.

FIG. 2 illustrates aligned receiver pads overlapping with transmitterpads.

FIG. 3 illustrates misaligned receiver pads overlapping with transmitterpads.

FIG. 4 illustrates two integrated circuit chips which are positionedface-to-face and aligned using etch pit wells in accordance with anembodiment of the present invention.

FIG. 5A illustrates an integrated circuit chip where the etch pitprocessing occurred after fabricating the circuits in accordance with anembodiment of the present invention.

FIG. 5B illustrates electrical traces inside the etch pit that establishconductive connections with other regions of the chip in accordance withan embodiment of the present invention.

FIG. 5C illustrates two integrated circuit chips containing electricalcircuits positioned face-to-face and conductively connected by a shapedstructure in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the claims.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. This includes, but is not limited to, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or any devicecapable of storing data usable by a computer system.

Proximity Communication

Proximity communication overcomes the limitations of resistive wires bycommunicating through capacitive coupling between chips that are placedface-to-face. As shown in FIG. 1, for an array of chips communicatingthrough proximity communication, the active faces 102 of the chipstypically face each other with full or partial overlap, with thecorresponding proximity communication pads 104 on opposing chips alignedboth laterally and vertically. The back faces 106 of chips are alsolabeled for clarity.

Proximity communication provides off-chip signaling bandwidth that canscale with the feature size and with the on-chip frequency. However,precise alignment constraints need to be satisfied for proximitycommunication to operate effectively. FIG. 3 illustrates a top-view oftwo integrated circuit chips where the receiver pads on one integratedcircuit chip are misaligned with the transmitter pads on the otherintegrated circuit chip. To effectively leverage the enormous bandwidthavailable through proximity communication, two chips need to be broughttogether and precisely aligned both laterally as well as axially (e.g.in the z-direction), so that the receiver pads align with thetransmitter pads as illustrated in FIG. 2. The mechanical alignmentmechanism needs to be reliable, manufacturable and cost-effective, andable to maintain alignment between the chips across six degrees offreedom.

Alignment Using Silicon Etch Pits

In one embodiment of the present invention, silicon etch pits aremicromachined into silicon integrated circuit chips. These micromachinedstructures can be formed photolithographically in sequence before,during, or after circuit fabrication, which allows the etch pits to bephotolithographically defined in relationship to the circuits. Thisphotolithographic alignment between the etch pits and circuits enablesthe top and bottom circuits to be precisely aligned with each other.

FIG. 4 illustrates two integrated circuit chips (402 and 404) which arepositioned face-to-face and aligned using etch pit wells (408 and 410)fabricated in the corners of each chip and a set of shaped structures,in this case spherical balls 406, that fit in the etch pits foralignment purposes. The spherical balls are inserted into the etch pitwells of the bottom chip before the positioning step. The two chips arethen first coarsely aligned mechanically. After the course alignment, aprecise alignment process begins in which the balls eventually co-locatethe two chips as the balls settle into the etch pits in each corner ofthe chips. At equilibrium, assuming that the spherical balls aresufficiently large to fit in the etched pit wells, the balls sit in theetch pit wells such that their equators lie higher than the chipsurfaces, and the top and the bottom circuits (414 and 412) alignprecisely.

FIG. 4 illustrates a situation where both the top and bottom chips havefull overlap. In another embodiment of the present invention, the topand bottom chips only overlap partially, but are still coupled andaligned with respect to each other using a substantially similartechnique. The depth and shape of the etch pit wells, as well as theshape and size of the shaped structures, can be finely tuned to optimizethe separation between the two chips for proximity communicationpurposes.

Alignment Using Conductive Structures

Two precisely-aligned integrated circuit chips can send data signals toeach other using proximity communication. However, transferring powerand ground signals to both chips typically requires a conductivechannel.

In one embodiment of the present invention, the lower chips in FIG. 1are referred to as “island chips” 108, and the upper chips are referredto as “bridge chips” 110. In such an arrangement, power and ground canbe externally provided to the island chips 108, while the bridge chips110 have less active circuitry and power consumption and serve primarilyto couple two or more island chips together. However, an even moreadvantageous and flexible configuration includes a way for the islandchips to also provide power and ground to the bridge chips.

One embodiment of the present invention extends the technique forface-to-face alignment of integrated circuit chips to use the sameshaped structures that provide alignment to also conduct power andground signals. These shaped structures provide a connector that isprecision-aligned both laterally and in relative height, and can bedisconnected and reconnected. The ability to disconnect and re-mate theshaped structure is an important improvement; while many structures canbe collapsed under pressure to provide a good conductive contact, suchstructures are not remateable.

The alignment structures placed in the etch pit wells can be comprisedof conductive materials, or can also be comprised of some othermaterial, such as glass or sapphire, with a conductive coating. Notethat it may be necessary to metallize the etch pit wells so that thecorresponding power and ground signals can be conducted from differentregions on the chip.

FIGS. 5A-5C illustrate the metallization of the etch pit wells. The etchpits can be made at the wafer scale, either before or after the circuitshave been fabricated. FIG. 5A illustrates an integrated circuit chipwhere the etch pit were made after fabricating the circuits 414. FIG. 5Billustrates electrical traces inside the etch pit that establishconductive connections with other regions of the chip. For instance, theelectrical traces might be created as an additional metal layer thatconnects to a region of a lower metal layer deliberately exposeddeliberately to allow a conductive connection. Redundancy can be usedboth for fault tolerance as well as to increase the amount of currentfor a signal. For instance, an etch pit well might be conductivelyconnected to a metal layer in two places using two sets of electricaltraces, to ensure the signal is transferred successfully. Alternatively,multiple conductive shapes and etch pit well pairs may be coupled to asingle electrical trace to increase the maximum current for a power orground signal.

FIG. 5C illustrates two integrated circuit chips containing electricalcircuits positioned face-to-face and conductively coupled by a shapedstructure, in this example a conductive ball 504. The etch pit wellscapture the conductive ball, which both provides a continuous path for apower or ground signal while also holding the two integrated circuitchips in precise mechanical alignment.

Note that many shaped structures of different size and type (withcorrespondingly shaped and sized etch pits) may be used to conduct a setof power and ground signals, while a smaller set of shaped structuresmay be needed to achieve alignment alone. A large number of shapedstructures might be used to uniformly distribute and share power andground between the chips. Shapes can include, but are not limited to,substantially spherical balls, truncated spheres, prisms, hexagons, andpyramids. Different shapes provide different advantages. For instance, aspherical ball may easily roll into an etch pit well despite beinginitially out of alignment, while if the point of a pyramid ispositioned anywhere in the etch pit during the initial coarse alignment,it will easily slide into and align with the etch pit well. Note alsothat using different shapes optimized either for alignment or conductionallows the fabrication tolerances for the position and orientation ofthe etch pits to be relaxed while still maintaining overall location andspacing requirements.

In one embodiment of the present invention, the shaped structures canalso vary in surface roughness and compliance. For surface roughness, avery fine surface provides the ultimate precision, but micro-roughnessmay allow better electrical contact. Such roughness is particularlyadvantageous when simultaneously used with compliant materials withinthe shaped structure or the etch pit wells (e.g. gold), or whenconductive materials such as a low-viscosity paste or liquid are usedwithin the wells to improve the surface contact area. Buildingcompliance into the shaped structure and etch pits also facilitatesbetter surface contact, and can be particularly beneficial to conductpower and ground between the chips. Compressive forces can be used tocreate compliance within the shaped structures or conductive elasticmaterials deposited within the wells, thereby increasing the contactsurface area and reducing the resistance of the contacts.

Additional Variations

In one embodiment of the present invention, the conductive shapedstructure is bonded in-place to the etch pit well of one of theintegrated circuit chips using conductive solder or epoxy, therebycreating a permanent low-resistance connection to the chip. Thistechnique creates a remateable male-female connector interface of thetype found in many connector topologies. Compliance in the shapedstructure can then be advantageously used on the female side to improvethe connection and reduce the resistance of the connection.

As mentioned previously, in additional embodiments of the presentinvention the top and bottom chips may overlap only partially, forinstance at the chip corners or along one or more sides of a chip, butstill align with each other using the described techniques.

In one embodiment of the present invention, tension is supplied duringthe passive alignment step and system operation by a compliant,suspension connector system that eliminates chip separation. Theconnector system provides sufficient compressive force on the chip tomaintain the shaped structures within the pits, thereby keeping thechips in alignment, while also providing sufficient compressive force toreduce the resistance of the conductive path between the chips. Thisconnector system is designed to provide some freedom of motion to allowthe connector chip to accurately mate with the fixed chip. Note that thecompressive force is sufficient to allow the final resting orientationto be determined solely by the orientation of the precision balls orspheres, while also assuring that the chips maintain proper spacing oncemated and that the resistance of the connection remains withinacceptable ranges. The connector system also provides some protectionfor the chips in the event there is some gross misalignment during themating of the two chips, so that there is some “give” that prevents aninterference fit and resulting damage to both chips. Since the precisionalignment and power connection are housing independent, low-costinjection-molded parts can be used for the connector system.

In summary, the techniques in the present invention describe aremateable connector that provides precision alignment betweensemiconductor chips while also transferring power and ground. Etchedfeatures in the semiconductor chips and precision conductive elementssimultaneously provide alignment and conduct power and ground signalsbetween the chips.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A method that facilitates precise inter-chip alignment for proximitycommunication and power delivery, comprising: placing a shaped structureinto an etch pit well located on the surface of a first integratedcircuit chip, wherein an etch pit well is an indented region in thesurface of an integrated circuit chip; substantially aligning acorresponding etch pit well located on the surface of a secondintegrated circuit chip with the shaped structure; and mating the shapedstructure with the etch pit well located on the surface of the secondintegrated circuit chip, thereby precisely aligning proximitycommunication structures in the first integrated circuit chip withcorresponding proximity communication structures in the secondintegrated circuit chip; wherein the shaped structure creates aconductive connection between the first integrated circuit chip and thesecond integrated circuit chip; and wherein multiple distinct conductiveelements with different locations in the etch pit well are used tocreate the conductive connection.
 2. The method of claim 1, wherein theetch pit well includes conductive material that conducts signals to andfrom other regions of the integrated circuit chip; and wherein a signalconducted between the first integrated circuit chip and the secondintegrated circuit chip through the shaped structure includes a powersignal or a ground.
 3. The method of claim 2, wherein a conductivematerial that reduces contact resistance is placed inside the etch pitwell to increase surface contact area and reduce electrical resistance;and wherein the conductive material includes a conductive low-viscositypaste or liquid.
 4. The method of claim 2, wherein the shaped structurecan be formed in a variety of different shapes that include: asubstantially spherical ball; a truncated sphere; a prism; a hexagon;and/or a pyramid.
 5. The method of claim 4, wherein multiple shapedstructures with different sizes are used; and wherein the multipleshaped structures can conduct different signals or a smaller set ofsignals with fault tolerance and/or higher current capacity.
 6. Themethod of claim 1, wherein the shaped structure is comprised of: aconductive material; or a non-conductive material with a conductivecoating.
 7. The method of claim 1, wherein substantially aligning thecorresponding etch pit well with the shaped structure involves applyingcompressive forces that align the first integrated circuit chip, theshaped structure, and the second integrated circuit chip; and whereincompliance in the shaped structure and/or the etch pit wells facilitatessurface contact between the shaped structure and the sidewalls or bottomof the etch pit wells.
 8. The method of claim 1, wherein the shapedstructure provides a connection that can be disconnected and reconnectedeasily.
 9. The method of claim 1, wherein the etch pit well and theshaped structure are formed using: an additive process; a subtractiveprocess; or both an additive and subtractive process.